This invention relates to organ transplantation.
Organ procurement currently poses one of the major problems in organ transplantation, as the number of patients requiring transplants far exceeds the number of organs available. Xenotransplantation may provide a solution to this problem. Phylogenetically, non-human primates are the most closely related species to humans and might therefore represent the first choice as donors. In 1969, Reemtsma et al., achieved the first successful kidney human xenograft from a chimpanzee (Reetsma, K. et al., 1964, Ann. Surg. 160:384). However, the potential utilization of primate donors is limited by insufficient numbers, legal and ethical considerations, and the potential for transmitting dangerous viral diseases. Swine represent one of the few large animal species in which breeding characteristics make genetic experiments possible, making it possible to develop MHC homozygous lines of miniature swine. Miniature swine can be maintained at maximum adult weights of 200 to 300 lbs and are anatomically and physiologically close to humans. Therefore the organs of miniature swine might be appropriate for use as xenografts for human beings of all ages.
Tolerance to self major histocompatibility (MHC) antigens occurs during T cell maturation in the thymus (McDuffie et al., 1988, J. Immunol. 141:1840). Exposure of the immune system to MHC antigens during ontogeny can cause the immune system to lose reactivity to those antigens, thus leaving the animal specifically tolerant into adult life (Billingham et al., 1953, Nature 172:603). Transplantation immunologists have sought means of inducing tolerance in adult animals by production of lymphohematopoietic chimeras. The induction of tolerance across MHC barriers in adult mice by whole body irradiation (WBI) and bone marrow transplantation (BMT) has been studied extensively in murine models (Rayfield et al., 1983, Transplan. 36:183; Mayumi et al., 1989, J. Exp. Med. 169:213; Sykes et al., 1988, Immunol. Today 9:23).
The use of MHC mismatched BMT as a means of inducing tolerance to organ grafts can be accompanied by several major disadvantages: the preparative regimen involves lethal irradiation, with its inherent risks and toxicities; clinical applicability is limited by the fact that most potential recipients do not have an appropriate MHC-matched donor, and BMT across MHC barriers causes severe graft-vs-host-disease (GVHD). Removing the T lymphocytes in allogeneic bone marrow inocula (Rodt et al., 1971, Eur. J. Immunol. 4:25) to prevent GVHD is associated with increased rates of engraftment failure (Martin et al., 1988, Bone Marrow Transplant 3:445; O'Reilly et al., 1985, Transplant. Proc. 17:455; Soderling et al., 1985, J. Immunol. 135:941). While these drawbacks are generally considered acceptable for the treatment of otherwise lethal malignant diseases, they would severely limit the application of this methodology as a preparative regimen for organ transplantation, in which non-specific immunosuppressive agents, while not without major complications, are effective.
Use of a relatively non-toxic, non-myeloablative preparative regimen for bone marrow engraftment and specific transplantation tolerance has been applied to the concordant rat to mouse species combination (Sharabi, Y. et al., 1990, J. Exp. Med. 172:195-202). The treatment involved administration of monoclonal antibodies to eliminate mature T cell subsets (CD4 and CD8) as well as NK cells (NK1.1). These monoclonal antibodies permitted engraftment of xenogeneic bone marrow after only a sub-lethal (300 rads) dose of WBI and a local dose of 700 rads thymic irradiation. The resulting lymphoid reconstitution was superior to that of previously mixed xenogeneic chimeras prepared by lethal irradiation and reconstitution with mixtures of T cell-depleted syngeneic and xenogeneic bone marrow (Sharabi, Y., et al., 1990, J. Exp. Med. 172:195-202; Ildstad, et al., 1984, Nature 307:168-170) as recipients did not suffer toxic effects from the preparative regimen. In addition, attempts have been made to lengthen the survival of skin allografts in primates and man by intravenously administering a polyclonal preparation of horse anti-human antithymocyte globulin (ATG). The ATG was injected simultaneously with and on days immediately following grafting (Cosimi, A. B., et al., 1970. Surgery. 68:54-61).
In discordant species combinations, the humoral (antibody mediated) component of the immune system poses a major barrier. When primarily vascularized organs are grafted between discordant species, natural antibodies that recognize determinants expressed on the surfaces of vascular endothelial cells cause rejection of the organ within minutes of vascular anastomosis, due to activation of the complement and coagulation cascades (Hammer, C., et al., 1973, Eur. Sug. Res. 5:162; Hardy, M. A. et al, 1984, in S. Slavin, ed. Elsevier, B. V., p. 515). In attempts to prolong cardiac xenografts from pig donors, pre-existing natural antibodies have been absorbed from the blood of recipient primates by hemoperfusion of a donor-specific kidney (Cooper, D. K. C., et al. 1988, J. Heart Transplan. 7:238-246; Fischel, R. J., et al., 1990, Transplant. Proc. 22:1077).